Assemble: ignore devices= if container= is present.

Assemble: ignore devices= if container= is present.

If "container=" is present, then we are going to assemble from the
given container where that container is made of those devices or not.
So in this case the "devices=" is purely documentation and is best
ignored.

As part of this, move the test on the "container=" value when that
start with "/" up before the device is opened.  There sooner we test
things, the better.

Reported-by: Martin Wilck <mwilck@arcor.de>
Signed-off-by: NeilBrown <neilb@suse.de>